The invention relates to a cordless telephone system comprising mobile sets and base stations, the base stations being connected via subscriber lines to a private automatic branch exchange, several mobile sets being allocated to one base station and subscriber numbers of the private automatic branch exchange being allocated to the mobile sets, the subscriber numbers being convertible by the base stations into the corresponding individual identifications of the mobile sets and the base stations being set up at different locations, the radio cells of the base stations covering different areas.
Such a telephone system is described in the report of the 37th IEEE Vehicular Technology Conference, 1-3 Jun. 1987, Tampa, Fla. (US) by T. Hattori et al. on page 579-586 under the title "A new mobile communication system using autonomous radio link control with decentralized base stations". The system requires a special radio control unit (RCU) which is connected between the branch exchange and the base stations. According to Section 5.1, the radio control unit contains a switching network in order to connect the exchange line to the base station in the radio cell of which the cordless subscriber is located.
Citation GB-A 2 193 861 describes a cordless telephone system having numerous base stations, the mobile sets in each case being allocated to a home base station for accounting purposes. At this point, the locations of cordless subscribers are collected by an access control unit. The search for a called subscriber is then started from the last known location and does not necessarily lead to the correct base station over a short path. Due to the hierarchical structure of local and regional control units, the search for the subscriber can heavily load the branch exchange.
In GB-A 2 166 622, a telephone system having several base stations is described which are combined to form one unit, several mobile sets being allocated to one base station. This combined station is connected to the private automatic branch exchange via one subscriber line per mobile set. Each mobile set selects a free frequency channel from a group and can set up a connection to a base station via this channel. However, one particular frequency channel is allocated to each base station. An incoming call for a cordless subscriber is switched by the private automatic branch exchange via the corresponding subscriber line of the mobile set to a free base station. The latter converts the subscriber number into the individual identification of the mobile set and signals the incoming call. At the same time, a next free base station is marked for further calls. An exchange of the individual identification ensures that connections are only set up between base stations and mobile sets of the same telephone system.
Cordless telephones in the 900-MHz band authorized by the postal authorities can be connected via an access unit to a private automatic branch exchange and act as extension like a conventional cord-type telephone set. Each mobile set forms one unit with its base station, which can be identified by individual exchange of identifications. The connection between mobile set and base station is set up via a free radio channel which is selected from 40 possible channels. A wrong connection between mobile set and base station is prevented by the fact that due to the individual identification, each mobile set can only be connected to its associated base station.
A further cordless telephone system is described in EP-A 180 178 in which several mobile sets are allocated to one base station. The mobile sets differ by their device-specific identification and several extension mobile sets are subordinated to one main mobile set. To make outgoing calls and receive incoming calls is only possible from the main mobile set. The extension mobile sets can only receive incoming calls. This prevents outgoing calls being made by possibly unsupervised mobile sets and the subscriber being charged.
In every case, the action radius of the subscriber with a mobile set is limited to about 100-200 meters since he leaves the radio range of the base station beyond that. Mobile telephones such as are used in the B- or C-network of the German post office have a basically wider radius of action. These mobile telephones consist of a relatively large and heavy transmitting and receiving section which has to be transported by the subscriber and is much more cumbersome than the mobile set of a cordless telephone. In these cellular mobile radio networks as, for example, in accordance with EP-A 260 763, the problem of the subscriber search does not occur in the form to be solved by the invention. Locating a mobile subscriber is effected continuously via a central communication channel and is centrally administered by an exchange. There are no stationary subscriber devices such as cord-type telephones in mobile radio networks.